Pixel Modules Research Articles

Module production of the one-arm AFP 3D pixel tracker

The ATLAS Forward Proton (AFP) detector is designed to identify events in which one or two protons emerge intact from the LHC collisions. AFP will consist of a tracking detector, to measure the momentum of the protons, and a time of flight system to reduce the background from multiple proton-proton interactions. Following an extensive qualification period, 3D silicon pixel sensors were selected for the AFP tracker. The sensors were produced at CNM (Barcelona) during 2014. The tracker module assembly and quality control was performed at IFAE during 2015. The assembly of the first AFP arm and the following installation in the LHC tunnel took place in February 2016. This paper reviews the fabrication process of the AFP tracker focusing on the pixel modules.

Optimization of thin n-in-p planar pixel modules for the ATLAS upgrade at HL-LHC

The ATLAS experiment will undergo around the year 2025 a replacement of the tracker system in view of the high luminosity phase of the LHC (HL-LHC) with a new 5-layer pixel system. Thin planar pixel sensors are promising candidates to instrument the innermost region of the new pixel system, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. The sensors of 50-150 μm thickness, interconnected to FE-I4 read-out chips, have been characterized with radioactive sources and beam tests. In particular active edge sensors have been investigated. The performance of two different versions of edge designs are compared: the first with a bias ring, and the second one where only a floating guard ring has been implemented. The hit efficiency at the edge has also been studied after irradiation at a fluence of 1015 neq/cm2. Highly segmented sensors will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. In order to reproduce the performance of 50x50 μm2 pixels at high pseudo-rapidity values, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angles with respect to the short pixel direction. Results on the hit efficiency in this configuration are discussed for different sensor thicknesses.

Thin hybrid pixel assembly with backside compensation layer on ROIC

The entire ATLAS inner tracking system will be replaced for operation at the HL-LHC . This will include a significantly larger pixel detector of approximately 15 m2. For this project, it is critical to reduce the mass of the hybrid pixel modules and this requires thinning both the sensor and readout chips to about 150 micrometres each. The thinning of the silicon chips leads to low bump yield for SnAg bumps due to bad co-planarity of the two chips at the solder reflow stage creating dead zones within the pixel array. In the case of the ATLAS FEI4 pixel readout chip thinned to 100 micrometres, the chip is concave, with the front side in compression, with a bow of +100 micrometres at room temperature which varies to a bow of −175 micrometres at the SnAg solder reflow temperature, caused by the CTE mismatch between the materials in the CMOS stack and the silicon substrate. A new wafer level process to address the issue of low bump yield be controlling the chip bow has been developed. A back-side dielectric and metal stack of SiN and Al:Si has been deposited on the readout chip wafer to dynamically compensate the stress of the front side stack. In keeping with a 3D process the materials used are compatible with Through Silicon Via (TSV) technology with a TSV last approach which is under development for this chip. It is demonstrated that the amplitude of the correction can be manipulated by the deposition conditions and thickness of the SiN/Al:Si stack. The bow magnitude over the temperature range for the best sample to date is reduced by almost a factor of 4 and the sign of the bow (shape of the die) remains constant. Further development of the backside deposition conditions is on-going with the target of close to zero bow at the solder reflow temperature and a minimal bow magnitude throughout the temperature range. Assemblies produced from FEI4 readout wafers thinned to 100 micrometres with the backside compensation layer have been made for the first time and demonstrate bond yields close to 100%.

Operational experience with the CMS pixel detector in LHC Run II

The CMS pixel detector was repaired successfully, calibrated and commissioned for the second run of Large Hadron Collider during the first long shutdown between 2013 and 2015. The replaced pixel modules were calibrated separately and show the expected behavior of an un-irradiated detector. In 2015, the system performed very well with an even improved spatial resolution compared to 2012. During this time, the operational team faced various challenges including the loss of a sector in one half shell which was only partially recovered. In 2016, the detector is expected to withstand instantaneous luminosities beyond the design limits and will need a combined effort of both online and offline teams in order to provide the high quality data that is required to reach the physics goals of CMS. We present the operational experience gained during the second run of the LHC and show the latest performance results of the CMS pixel detector.

Investigation of thin n-in-p planar pixel modules for the ATLAS upgrade

In view of the High Luminosity upgrade of the Large Hadron Collider (HL-LHC), planned to start around 2023–2025, the ATLAS experiment will undergo a replacement of the Inner Detector. A higher luminosity will imply higher irradiation levels and hence will demand more radiation hardness especially in the inner layers of the pixel system. The n-in-p silicon technology is a promising candidate to instrument this region, also thanks to its cost-effectiveness because it only requires a single sided processing in contrast to the n-in-n pixel technology presently employed in the LHC experiments. In addition, thin sensors were found to ensure radiation hardness at high fluences. An overview is given of recent results obtained with not irradiated and irradiated n-in-p planar pixel modules. The focus will be on n-in-p planar pixel sensors with an active thickness of 100 and 150 μm recently produced at ADVACAM. To maximize the active area of the sensors, slim and active edges are implemented. The performance of these modules is investigated at beam tests and the results on edge efficiency will be shown.

Development of a Detector Control System for the ATLAS Pixel detector in the HL-LHC

The upgrade of the LHC to the HL-LHC requires a new ITk detector. The innermost part of this new tracker is a pixel detector. The University of Wuppertal is developing a new DCS to monitor and control this new pixel detector. The current concept envisions three parallel paths of the DCS. The first path, called security path, is hardwired and provides an interlock system to guarantee the safety of the detector and human beings. The second path is a control path. This path is used to supervise the entire detector. The control path has its own communication lines independent from the regular data readout for reliable operation. The third path is for diagnostics and provides information on demand. It is merged with the regular data readout and provides the highest granularity and most detailed information. To reduce the material budget, a serial power scheme is the baseline for the pixel modules. A new ASIC used in the control path is in development at Wuppertal for this serial power chain. A prototype exists already and a proof of principle was demonstrated. Development and research is ongoing to guarantee the correct operation of the new ASIC in the harsh environment of the HL-LHC. The concept for the new DCS will be presented in this paper. A focus will be made on the development of the DCS chip, used for monitoring and control of pixel modules in a serial power chain.

The FE-I4 telescope for particle tracking in testbeam experiments

A testbeam telescope, based on ATLAS IBL silicon pixel modules, has been built. It comprises six planes of planar silicon sensors with 250 × 50 μ m2 pitch, read out by ATLAS FE-I4 chips. In the CERN SPS H8 beamline (180 GeV π+) a resolution of better than 8 × 12 μm2 at the position of the device under test was achieved. The telescope reached a trigger rate of 6 kHz with two measured devices. It is mainly designed for studies using FE-I4 based prototypes, but has also been successfully run with independent DAQ systems. Specialised trigger schemes ensure data synchronisation between these external devices and the telescope. A region-of-interest trigger can be formed by setting masks on the first and the last pixel sensor planes. The setup infrastructure provides centrally controlled and monitored high and low voltage power supplies, silicon oil cooling, temperature and humidity sensors and movable stages.

Thin hybrid pixel assembly fabrication development with backside compensation layer

The ATLAS and CMS experiments will both replace their entire tracking systems for operation at the HL-LHC in 2026. This will include a significantly larger pixel systems, for example, for ATLAS approximately 15m2. To keep the tracker material budget low it is crucial to minimize the mass of the pixel modules via thinning both the sensor and readout chip to about 150μm each. The bump yield of thin module assemblies using solder based bump bonding can be problematic due to wafer bowing during solder reflow at high temperature. A new bump-bonding process using backside compensation on the readout chip to address the issue of low yield will be presented. The objective is to compensate dynamically the stress of the front side stack by adding a compensating layer to the backside of the wafer. A SiN and Al:Si stack has been chosen for the backside layer. The bow reducing effect of applying a backside compensation layer will be demonstrated using the FE-I4 wafer. The world's first results from assemblies produced from readout wafers thinned to 100μm with a stress compensation layer are presented with bond yields close to 100% measured using the FE-I4 readout chip.

Thin n-in-p planar pixel modules for the ATLAS upgrade at HL-LHC

The ATLAS experiment will undergo a major upgrade of the tracker system in view of the high luminosity phase of the LHC (HL-LHC) foreseen to start around 2025. Thin planar pixel modules are promising candidates to instrument the new pixel system, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. New designs of the pixel cells, with an optimized biasing structure, have been implemented in n-in-p planar pixel productions with sensor thicknesses of 270μm. Using beam tests, the gain in hit efficiency is investigated as a function of the received irradiation fluence. The outlook for future thin planar pixel sensor productions will be discussed, with a focus on thin sensors with a thickness of 100 and 150μm and a novel design with the optimized biasing structure and small pixel cells (50×50 and 25×100μm2). These dimensions are foreseen for the new ATLAS read-out chip in 65nm CMOS technology and the fine segmentation will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. To predict the performance of 50×50μm2 pixels at high η, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angle with respect to the short pixel direction. Results on cluster shapes, charge collection- and hit efficiency will be shown.

Advanced alignment of the ATLAS tracking system

In order to reconstruct the trajectories of charged particles, the ATLAS experiment exploits a tracking system built using different technologies, silicon pixel modules or microstrips and gaseous drift tubes, all embedded in a 2T axial magnetic field. Misalignments of the active detector elements and deformations of the structures (which can lead to Weak Modes) deteriorate resolution of the track reconstruction and lead to systematic biases on the measured track parameters. The applied alignment procedures exploit various advanced techniques in order to minimise trackhit residuals and remove detector deformations. For the LHC Run II, the Pixel detector has been refurbished and upgraded with the installation of a new pixel layer, the Insertable B-layer.

Performance of silicon pixel detectors at small track incidence angles for the ATLAS Inner Tracker upgrade

In order to enable the ATLAS experiment to successfully track charged particles produced in high-energy collisions at the High-Luminosity Large Hadron Collider, the current ATLAS Inner Detector will be replaced by the Inner Tracker (ITk), entirely composed of silicon pixel and strip detectors. An extension of the tracking coverage of the ITk to very forward pseudorapidity values is proposed, using pixel modules placed in a long cylindrical layer around the beam pipe. The measurement of long pixel clusters, detected when charged particles cross the silicon sensor at small incidence angles, has potential to significantly improve the tracking efficiency, fake track rejection, and resolution of the ITk in the very forward region. The performance of state-of-the-art pixel modules at small track incidence angles is studied using test beam data collected at SLAC and CERN.

Development of n-in-p pixel modules for the ATLAS upgrade at HL-LHC

Thin planar pixel modules are promising candidates to instrument the inner layers of the new ATLAS pixel detector for HL-LHC, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. 100–200μm thick sensors, interconnected to FE-I4 read-out chips, have been characterized with radioactive sources and beam tests at the CERN-SPS and DESY. The results of these measurements are reported for devices before and after irradiation up to a fluence of 14×1015neq/cm2. The charge collection and tracking efficiency of the different sensor thicknesses are compared. The outlook for future planar pixel sensor production is discussed, with a focus on sensor design with the pixel pitches (50×50 and 25×100 μm2) foreseen for the RD53 Collaboration read-out chip in 65nm CMOS technology. An optimization of the biasing structures in the pixel cells is required to avoid the hit efficiency loss presently observed in the punch-through region after irradiation. For this purpose the performance of different layouts have been compared in FE-I4 compatible sensors at various fluence levels by using beam test data. Highly segmented sensors will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. In order to reproduce the performance of 50×50μm2 pixels at high pseudo-rapidity values, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angle (80°) with respect to the short pixel direction. Results on cluster shapes, charge collection and hit efficiency will be shown.

CMS pixel upgrade for the phase I: Module production and qualification

The present CMS pixel detector has been designed to be fully efficient up to an LHC luminosity of 1034cm−2s−1. However, the luminosity will increase by a factor of two in the coming years. Therefore it is planned to build and install a new detector in the extended year-end technical stop in 2016–17. Barrel pixel modules are in production since spring/summer 2015 in five different centers. Module production requires bump bonding, wire bonding and gluing processes to finally assemble a full module. To have a uniform performance of all modules standard qualification procedures have been developed. All modules will be subjected to 10 thermal cycles between +17°C and −25°C and then electrically tested. In addition, module performance will be verified under high rate X-rays, and internal calibrate signals used for electrical tests will be calibrated in units of electrons using well defined X-ray fluorescence lines from different target materials. The qualification criteria, based on which modules are selected to be used in the final system, will be explained in detail.

Experience from design, prototyping and production of a DC-DC conversion powering scheme for the CMS Phase-1 Pixel Upgrade

The CMS collaboration has adopted a DC-DC conversion powering scheme for the Phase-1 Upgrade of its pixel detector. DC-DC buck converters with a conversion ratio of around 3 are installed on the support structures, outside of the sensitive tracking region, requiring a re-design of the low and high voltage distribution to the pixel modules. After several years of R&D, the project has entered the production phase. A total of 1800 DC-DC converters are being produced, and rigorous quality assurance and control is being employed during the production process. The testing program is outlined, results from mass production are presented and issues that have been encountered are described. In addition, two system level challenges, namely the choice of output voltage in the presence of large, load-dependent voltage drops, and the thermal management required to remove the heat load caused by the DC-DC converters, are discussed.

CMOS Global Shutter Charge Storage Pixels With Improved Performance

We describe different charge-storage-based global shutter (GS) pixel architectures with improved performance resulting from low dark current and effective pixel pitch reduction from 6 to 2.8 $\mu \text{m}$ pixel size. Smaller pixels showed better image quality versus larger pixels at low exposures and higher temperatures. In addition to earlier disclosed work, we proposed and have developed novel high GS efficiency (GSE) pixel modules such as the improved tungsten buried light shield and the pixel light guide structures. The GS pixel simulation and measurements showed significant improvement in sensitivity, dark current, and GSE. These pixels and modules have been implemented in a variety of ON Semiconductor sensors. More than $1.2\times $ area-scaled sensitivity improvements, more than $20\times $ improvement in average storage dark current, and from $7\times $ to more than $38\times $ GSE improvement at different light wavelengths were demonstrated.

The pixel tracking telescope at the Fermilab Test Beam Facility

An all silicon pixel telescope has been assembled and used at the Fermilab Test Beam Facility (FTBF) since 2009 to provide precise tracking information for different test beam experiments with a wide range of Detectors Under Test (DUTs) requiring high resolution measurement of the track impact point. The telescope is based on CMS pixel modules left over from the CMS forward pixel production. Eight planes are arranged to achieve a resolution of less than 8μm on the 120GeV proton beam transverse coordinate at the DUT position. In order to achieve such resolution with 100×150μm2 pixel cells, the planes were tilted to 25 degrees to maximize charge sharing between pixels. Crucial for obtaining this performance is the alignment software, called Monicelli, specifically designed and optimized for this system. This paper will describe the telescope hardware, the data acquisition system and the alignment software constituting this particle tracking system for test beam users.

Overview and present status of the CMS Phase 1 pixel upgrade

During Run 2 of the LHC a significant luminosity increase to 2×1034cm−2s−1 is foreseen. As the innermost tracking device of CMS, the silicon pixel detector has to cope with large particle fluxes and radiation damage. To maintain the present high tracking efficiency, the current pixel detector will be replaced during an extended winter shutdown in 2016/2017. The design of the new detector is described, with a special focus on the construction and testing of the pixel modules.

Holography using pixelated spatial light modulators—part 1: theory and basic considerations

Pixelated spatial light modulators are used today in large quantities for projecting information. Apart from this standard application, the elements have found widespread use as dynamic changeable holograms for a multitude of applications ranging from microscopy over optical measurement systems to lithography. In this first part of our contribution, we give an overview concerning the necessary theory for the successful application of pixelated modulators in holography. We focus on the limitations imposed by current and future technology.

Holography using pixelated spatial light modulators—Part 2: applications

Dynamic holography can replace or enhance traditional basic optical functionalities within a lot of different optical systems. In particular, the possibility of considerably changing the function of a system is an important strength that is rarely possible in conventional optical systems. Today, different spatial light modulators can be employed in order to achieve dynamic holography, but most often liquid crystal on silicon phase modulators are used. We give an overview of applications for the usage of pixelated spatial light modulators in dynamic holography based on the functionalities that the dynamic holograms fulfill.

Irradiation and testbeam of KEK/HPK planar p-type pixel modules for HL-LHC

For the ATLAS detector upgrade for the high luminosity LHC (HL-LHC), an n-in-p planar pixel sensor-module is being developed with HPK. The modules were irradiated at the Cyclotron RadioIsotope Center (CYRIC) using 70 MeV protons. For the irradiation, a novel irradiation box has been designed that carries 16 movable slots to irradiate the samples slot-by-slot independently, to reduce the time for replacing the samples by hand, thus reducing the irradiation to human body. The box can be moved horizontally and vertically to scan the samples for a maximum area of 11 cm × 11 cm. Tests were subsequently carried out with beam at CERN by using 120 GeV pions and at DESY with 4 GeV electrons. We describe the analyses of the testbeam data of the KEK/HPK sensor-modules, focussing on the comparison of the performance of old and new designs of pixel structures, together with a reference of the simplest design (no biasing structure). The novel design has shown comparably good performance as the no-structure design in detecting passing-through charged particles.